1. Field of the Invention
The present invention relates to an extruded stretched filament of ultra-high-molecular-weight polyethylene and to a method and apparatus for producing the same. The present invention also relates to a whitened wire of ultra-high-molecular-weight polyethylene capable of being plastically deformed into any desired shape and a method for producing the same.
2. Description of the Related Art
Ultra-high-molecular-weight polyethylene is increasingly used as a so-called engineering plastic because of its excellent impact resistance, abrasion resistance, chemical resistance, tensile strength, and other properties, when compared with conventional polyethylene. However, ultra-high-molecular-weight polyethylene has an extremely high melt viscosity and poor flowability when compared with conventional polyethylene, and thus it is very difficult to mold such a plastic by conventional extrusion molding and injection molding. Accordingly, ultra-high-molecular-weight polyethylene is generally molded by compression molding; with the exception of rods and the like, which are partly extrusion molded at a very low extrusion speed.
On the other hand, as a method for stretching the monofilament of high-density polyethylene at a high draw ratio, it has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) No. 37-9765, that an additive (e.g., o-chlorobenzene, xylene) having a boiling point higher than the melting point of the polyethylene is incorporated into the polyethylene in an amount of 20% to 150% by weight based on the weight of the polyethylene and the resultant dispersion is formed into a primary fibrous material, followed by hot stretching at a draw ratio of 3 to 15 while retaining the 5% to 25% by weight corresponding amount of the additive in the spun threads. It has been also proposed in, for example, Japanese Unexamined Patent Publication No. 56-15408, that a solution of linear polyethylene having a molecular weight of 400,000 or more is spun, followed by stretching the same at a temperature such that the modulus of at least 20 GPa can be obtained. However, although these methods can provide stretched filaments having a high elasticity and high strength, raw filaments having a diameter of more than 1 mm.phi. are very difficult to obtain since a solution having an extremely low viscosity obtained by diluting ultra-high-molecular-weight polyethylene or the like with a large amount of a solvent is extruded and processed. Thus, the stretched thread generally has a thickness of 10 deniers or less (i.e., diameter: 0.038 mm.phi. or less) and a thickness of at most about 50 deniers. Accordingly, since a large amount of these stretched fine threads must be interwined to form thick ropes and nets having a high strength, or cords for cord cutters (i.e., lawn mowers), there are problems in that the process operations are troublesome, the stiffness of the resultant ropes and nets is unpreferably high, and the flexibility is insufficient. Furthermore, the resultant ropes and nets are usually very hairy.
In addition, it has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 52-74682, that polymers having a weight-average molecular weight of 150,000 or more are processed under the conditions of a temperature of about 75.degree. C. to about 140.degree. C. and a deformation ratio of about 18 or more to form elongated and orientated polymers. However, this reference describes that the diameter of fibers or the thickness of films or tapes before stretching is preferably about 1 mm or less. Also, the Examples given therein only disclose that polyethylene having a molecular weight of about 300,000 is spun through circular orifices having a diameter of 1 mm to form thin filaments having a diameter of 0.7 mm or less and, in the case of polyethylene having a molecular weight of 800,000, filaments are prepared only by stretching dumbbell-shaped samples having a gauge dimension of 1 cm .times.0.2 cm derived from the compressed sheet having a thickness of 0.5 mm. Thus, it has not been established in the art that filaments having a diameter of more than 1 mm.phi. can be industrially produced from ultra-high-molecular-weight polyethylene having an extremely high molecular weight.
Furthermore, it has been desired in the art to substitute plastic wires for the iron wires usually used in, for example, orchards, e.g., for tying the loculi of fruit and for grape-vine trellises, because iron wires easily rust and cannot withstand the effects of long-term use. However, conventionally commercialized plastic wires composed of nylon and polypropylene have a certain elasticity and, therefore, cannot be plastically deformed, unlike iron wires: For example, plastic wires cannot be firmly joined only by twisting.